1. Field of the Invention
This invention is concerned with a rotary servo valve for improving the high-frequency power output and for shaping the output signal of a seismic vibrator.
2. Description of the Prior Art
There are many types of seismic vibrator systems available for use in shaking the earth for geophysical exploration. Typically a seismic vibrator consists essentially of a large base plate for contacting the earth. A hydraulic linear actuator reciprocates the base plate relative to a reaction mass at selected frequencies to inject a desired vibratory wave train into the earth. A typical vibrator is shown, for example, in U.S. Pat. No. 3,745,885.
The linear actuator usually consists of a double-faced drive piston mounted concentrically on a shaft. The lower end of the shaft is connected to the base plate. The shaft and drive piston are enclosed within a double-acting hydraulic cylinder formed within the reaction mass. Hydraulic fluid under pressure is applied alternately, through suitable plumbing, to the hydraulic cylinder on opposite sides of the drive piston to reciprocate the linear actuator. The entire assembly is usually carried by a suitable vehicle. During transport from place to place the vibrator assembly is lifted clear of the ground. In operation, the base plate is pressed against the ground by the static weight of the carrier vehicle which is generally applied through isolator springs such as air bags.
The hydraulic system used to drive the linear actuator consists first of an electrohydraulic pilot valve. The pilot valve is electrically driven by a suitably-programmed pilot signal such as a chirp signal whose frequency varies monotonically with time. A typical chirp signal might include a three-octave spectrum from 10 to 80 Hertz (Hz) over a period of, perhaps, 16 seconds. Special chirp signals extending an additional octave to 160 Hz may be used to increase system resolution.
The hydraulic output of the pilot valve drives a hydraulic power stage servo valve which in turn, drives the hydraulic linear actuator. An accelerometer mounted on the base plate monitors the motion of the baseplate. Through a feedback loop, the accelerometer output signal is used to adjust the input signal to the pilot valve so that the signal actually injected into the earth remains in proper phase relationship with the reference signal. Examples of known hydraulic circuits may be found in any one of a large number of vibrator patents such as U.S. Pat. No. 3,881,167 or in U.S. patent application No. 255,205 filed Mar. 17, 1981 and assigned to a related firm of the assignee of this invention.
In all of the known seismic vibrators, the pilot and power stage valves are conventional, linear, spool-type servo valves. In the power stage servo, the spool reciprocates back and forth to open first one valve port and then the other so that hydraulic fluid flows first to one side and then to the other side of the double-faced drive piston.
The spool of the power stage servo has substantial inertia. At the higher frequencies of the chirp signal or sweep, the inertia of the spool tends to constrain the spool from fully opening its valve ports during each stroke of the spool. The result is a substantial loss of power, due to the restricted fluid flow to the linear actuator, at sweep frequencies above about 100 Hz. The base plate amplitude loss at high frequencies relative to the lower frequencies may become as large as 24 dB per octave or more.
Another problem associated with known vibrator systems, particularly at lower sweep frequencies, involves hydraulic-fluid pressure surges. As mentioned before, at low frequencies hydraulic fluid is applied, at full pressure, alternately to each of the opposite sides of the drive piston of the linear actuator to reciprocate the base plate relative to the mass. As the actuator is completing its stroke in one direction full hydraulic power is still being applied to the powered side of the drive piston. As the stroke ends, the fluid flow is abruptly reversed to the opposite side of the piston for the return stroke. Due to inertia of the linear actuator the drive piston unavoidably collides momentarily with the suddenly locked oil column entrapped in the cylinder and then with reversed inflow of pressurized fluid, creating a severe hydraulic shock or "water hammer" effect.
The hydraulic shock generates unwanted spikes in the output signal and wreaks havoc with the hydraulic-line hoses and plumbing. The problem above outlined becomes more serious as the chirp signal frequency decreases because of the higher velocity of actuator motion relative to the reaction mass. To prevent physical system damage, the power output of the vibrator must be attenuated by the operator to a safe but undesired lower level.
Thus, because of system inertia, both high and low frequency response and available output power are limited when using known vibrator system designs that require use of a spool type power stage servo valve.